Homopolymers and copolymers of unsaturated carboxylic acids, such as acrylic acid, methacrylic acid, itaconic acid, maleic acid with other vinylidene monomers, are well known, as described in patent U.S. Pat. No. 2,798,053 and in subsequent patents on the same matter.
For example, subsequent patents U.S. Pat. No. 4,375,533, U.S. Pat. Nos. 4,419,502 and 4,996,274 disclose polymerizations carried out in different solvent systems, such as aromatic, hydrocarburic, halogenated, wherein said unsaturated acids polymerize by precipitation in the form of fine powders, which are subsequently dried, in order to be used as thickening and suspending agents in different industrial fields.
The solvents and monomers used in said patents however are toxic, and residue quantities in the obtained products hinder their use in the cosmetic and pharmaceutical field.
The possibility of using each of the solvents having a hydrogen bond number between 0.7 and 1.7 and solubility parameters between 8 and 15 is claimed in U.S. Pat. No. 4,267,103, in particular ethyl acetate is suggested when more than 3% by weight of monomer acids in the monomer mixture is in neutral form.
U.S. Pat. No. 4,758,641 claims the use of acetone and alkyl acetates, already cited in U.S. Pat. No. 4,267,103, used for the polymerization, but with a water content limited to 1% by weight to reduce the content of monomer residue under 0.2% in order to lower the toxicity of the finished product. Said solvents have a low grade of toxicity: nevertheless they are difficult to remove from the finished product which generally contains about 0.8% by weight of them, unless it is treated with prolonged drying systems at high temperature, whereby the polymer is degraded causing the formation of unwanted by-products.
Relying on the state of the art, then it is impossible to produce a polymer such as polyacrylic acid with a residue monomer content lower than 0.2% by weight and with a low-toxicity solvent residue whose content is below 0.8% by weight.